The present invention pertains to portable communication devices and more particularly to a position aided, portable communication device for use with a satellite communication system.
When achieving communication between satellites and users of a satellite communication system around the world, knowledge of the location of each user is important to establishing and maintaining the communication link. Knowledge of the location of a particular user is also important because that location information may be reported through the satellite link which serves the user to any other user worldwide. A single unit which combines voice/data communication and precise location determination has several advantages over the present day communication system devices. Applications of such a system include asset management as well as coordination of activities of search and rescue and various military operations.
Since communications of the user unit to satellites involve frequent hand-offs between cells created by individual satellites and between adjacent satellites, the knowledge of precise location of both the user unit and of the satellites can be used to efficiently determine an appropriate hand-off strategy.
Further, precise knowledge of the location of a user unit also aids in the acquisition process by permitting precise correction for Doppler and reference frequency induced errors. The satellites of a low-earth orbit satellite communication system such as Iridium move very rapidly compared with the location of a user on earth. During communications, the system must provide Doppler compensation to the signals to compensate for the rapidly changing propagation delay in the link. During acquisition of the signals, if the user unit cannot independently determine the Doppler frequency, an exhaustive frequency search of the Doppler range must be performed. This length of time for an exhaustive search would delay the recognition of the user by the system. Therefore, in the case of telecommunication, the user would experience a delay in being able to establish communications.
However, the communication satellite ephemerides can be stored in the user unit and will remain valid for several days. Within knowledge of the precise location of the user unit, the Doppler adjustment as well as the correction to the reference oscillator can be explicitly computed. This would permit more rapid acquisition and tracking of the satellite pilot channel.
Accordingly, it is an object of the present invention to provide a combined position aided voice/data communicator device for use with a satellite communication system.